Just... The fact that Yuuko's been wandering around the school, completely invisible, and when seen at all she's seen as a horrible monster, for decades. It's no wonder she becomes so attached to Niiya so quickly. He's the only one she has.
Then there's Yukariko, who has been living with the guilt of killing her own sister for all that time. It probably doesn't help that Kirie looks so much like Yuuko.
Episode 10. All of episode 10 is either this or Nightmare Fuel. You get to see Yuuko before the human sacrifice, and she's one of the nicest people you could hope to meet. Butthen...
More so if you think about it from Teiichi's perspective. It's not been long since he confessed his feelings for Yuuko and then is powerless whilst watching the very answers they've been seeking playing out in front of him. He tries to stop her from effectively walking into a trap, but has no power to change Yuuko's memories. By the time she is selected as the Human Sacrifice, he looks emotionally drained and can only speak words of sympathy in response to Yuuko's pains, none of which will get through to her. Again, it doubles as Nightmare Fuel as he's watching the girl he loves suffering and dying.
If you think a little deeper about it, it gets even worse. He was there the whole time. The whole time, and could feel everything she felt. Remember, she didn't die of blood loss, nor was she Mercy Killed. It's pretty clear from her physical appearance (the dried and drawn back lips, the shaking, etc) that she was dying of starvation and thirst, and that he had to watch (and feel) the love of his life die that long, slow death, while dealing with a broken leg that has a a bone sticking out of it for days. Dying of starvation and thirst on its own is incredibly painful, dealing with a broken leg, with a bone sticking out of itat the same time... On top of that, you have what's mentioned above about not being able to do anything about it, or even give her any form of comfort as she lay there, dying, alone, literally finding out what she is in the dark, and almost giving in.
Last 1/4th of episode 12. Yuuko finally disappears. The music didn't help any.
Then she reappears.
In the manga, Yukariko having a change of heart too late and falling down the stairs right into her sister's lap. Then being so happy when the vengeful ghost tries to strangle her. Which Yuuko ultimately couldn't, because beneath the hate Yuuko still loved her sister too much to do it.
Yuuko's disappearance is in so many ways much worse in the anime than in the manga.
Anime!Yuuko doesn't want Keiichi to watch her disappear, so she begs him to leave before she does. Keiichi refuses, so they waste a lot of their last time together arguing back and forth. Manga!Yuuko disappears much quicker, so she barely has time to reach out for Keiichi before fading away.
Since Anime!Yuuko disappears gradually rather than instantly, the couple spends their last moment visiting the place they first met. Mid-way through, however, Yuuko's voice disappears. Keiichi can only communicate with her through a notebook. Soon after, her visage disappears, leaving Keiichi alone with only her writing as the last bit of her presence. The scene ends with the invisible Yuuko giving Keiichi a kiss. As soon as she vanishes completely, Keiichi is left desperately screaming out her name. The song requiem makes the scene so much more powerful.
